It is often desired to know the color of light emitted by a light source (e.g., a visual display, illumination system, or any other surface or volume that emits, transmits, or reflects optical radiation) in order to characterize its performance. The measured spectral data may be mathematically analyzed to determine the color of the light defined by a standard color space. Commonly used color spaces for illumination systems include, for example, the (a) CIE (Commission Internationale de l'Eclairage) 1931 (x, y) chromaticity coordinates, and (b) CIE 1964 (u′, v′) chromaticity coordinates.
One conventional process for testing visual displays or other light sources includes taking “spot” measurements with a spectroradiometer at a number of different portions of the display. A spectroradiometer, which is generally the most accurate instrument for determining the color of light in quantifiable units, captures the power per wavelength of visible light emitted by a light source, denoted by S(λ). The spectroradiometer is able to calculate the CIE 1931 tristimulus values by employing the following formulas defined by CIE:
                    X        =                              ∫                          380              ⁢                                                          ⁢              nm                                      780              ⁢                                                          ⁢              nm                                ⁢                                    S              ⁡                              (                λ                )                                      ⁢                          x              ⁡                              (                λ                )                                      ⁢                          ⅆ              λ                                                          (        1        )                                Y        =                              ∫                          380              ⁢                                                          ⁢              nm                                      780              ⁢                                                          ⁢              nm                                ⁢                                    S              ⁡                              (                λ                )                                      ⁢                          y              ⁡                              (                λ                )                                      ⁢                          ⅆ              λ                                                          (        2        )                                Z        =                              ∫                          380              ⁢                                                          ⁢              nm                                      780              ⁢                                                          ⁢              nm                                ⁢                                    S              ⁡                              (                λ                )                                      ⁢                          z              ⁡                              (                λ                )                                      ⁢                          ⅆ              λ                                                          (        3        )            In the above formulas, x(λ), y(λ) and z(λ) are the CIE 1931 2 degree standard observer color matching functions specifically defined by the CIE to serve as international definitions for the CIE 1931 (x, y) color space. The chromaticity coordinates are then determined by the following formulas:
                    x        =                  X                      X            +            Y            +            Z                                              (        4        )                                y        =                  Y                      X            +            Y            +            Z                                              (        5        )            
If the spectroradiometer has been accurately calibrated to measure the spectral content of light, there is no further calibration required of a spectroradiometer to measure the CIE 1931 tristimulus values and precisely calculate the 1931 chromaticity coordinates (x, y). However, despite the fact that spectroradiometers can generally provide accurate results when testing light sources, such processes are far too slow for production applications where many individual spot measurements are required, and are generally unable to measure a large area.
Another approach for measuring visual displays or other light sources includes the use of an imaging photometer (e.g., a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) imaging system). A CCD system, for example, can substantially improve the speed and efficiency of testing such displays because a CCD array can include a large number of detectors and, accordingly, a single measurement with the CCD system can include a very large number of test points (e.g., in excess of 1×106). Rather than taking many separate spot measurements of a visual display (as necessary with the above-described spectroradiometer), the CCD system can image the entire display simultaneously and capture many thousands or millions of points in just seconds.
Measuring displays with CCD systems, however, also includes several drawbacks. CCD imaging systems generally use several color filters (e.g., three or more filters, usually including red, green, and blue filters) through which light is measured. A color calibration (e.g., a matrix or other set of correction factors) is then applied to the measured filtered light to calculate the measured values in terms of tristimulus values, which correlates to the human eye response. Typically, the transmission of these filters, in combination with the camera and lens response, does not exactly match the tristimulus curves. One way to overcome this problem is to try to include additional color filters. This method can increase the accuracy of the camera, but only for the spectrum that the additional curves measure. For example, a color filter that is centered only in a narrow red region cannot help in the blue region. Additionally, this method adds a costly component to the system and can substantially slow down the measurement process. Accordingly, there is a need to improve the systems and methods for determining color calibrations for image measurements.